In addition to sending voice and/or data over a telephone wireline pair, a telecommunication service provider may use the same pair of wires to supply electrical power to various utility devices/circuits, including, but not limited to repeaters that are distributed along the wireline, and terminal devices that remotely terminate the line. The power used to operate such equipment via the wireline pair is customarily sourced from the telephone company's central office (CO). This method of powering equipment from the line is commonly referred to as ‘loop’- or ‘span’-powering, and enables the phone companies to remotely power equipment installed up to several miles from the central office.
Although DC voltage and power requirements for a line-powering supply (part of the central office terminal) are well understood, characteristics that ensure successful start-up of the multiple elements that make up the system have not been fully appreciated to date, due in part to the fact that there are numerous modes of start-up failure. Auspiciously, start-up failure of line-powered systems has not been a significant problem in the past. Moreover, in general, it has not been difficult to construct multi-element line-powered systems, which are effectively free of start-up problems. In addition, even when start-up problems were evident, they were typically not difficult to resolve.
Unfortunately, this apparent ease of span-powered system implementation has allowed system designers to avoid having to fully understand the requirements for successful start-up and sustained operation of a multi-element span-powered system. Factors that have facilitated the implementation of such networks include the time delaying function of the input capacitance of the line-powered elements (which delays the voltage across line elements as the span voltage rises) and the delay time or ramp-up time in the power consumed by a line-powered element (which lowers the span voltage requirements to achieve successful start-up).
Despite the fact that line-powered, multiple element start-up has not been a pervasive problem to date, in order to be able to specify inter-operability of terminal equipment from multiple manufacturers, and thereby comply with projected industry standards, start-up requirements must be understood and quantified.